In the prior art adjustable spanner, when a nut has rotated through a predetermined angle, the adjustable spanner must leave from the nut and returns to the original position for further driving the nut. Furthermore, the operation is performed repeatedly so as to achieve the object of screwing or releasing the nut.
In one prior art, a retaining jaw of the adjustable spanner is a receiving chamber having an opening. A driving roller with two posts at two ends is received within the receiving chamber. Two cover sheets cover the two end surfaces of the retaining jaw. The two posts are resisted by a torsional spring in the retaining jaw. Normally, the driving roller protrudes from the receiving chamber. When the adjustable spanner rotates a nut (stud) and the driving roller suffers from an external force, the driving roller moves along a cambered sliding track formed by the two covers, and moves inwards along the track. Thereby the adjustable spanner rotates reversely without leaving from the nut and then drives the nut (or stud).
Above mentioned structure can lock or release a nut or a stud easily and quickly. However, the prior art has the following disadvantages. Firstly, it has the complicated structure. The two surfaces of the retaining jaw must be cut to form a receiving chamber with three open ends. Furthermore, the track is formed by the two covers for confining the two posts. The covers must be formed with sector slots so that when the driving roller moves inwards, the elastomer can be adhered in the sector slot so that the elastomer can effectively resist against the driving roller. Thereby the structure is complicated.
It is difficult in the manufacturing process because to form the sector slot is time tedious and needs much works. Thereby the cost is increased for forming the sector slots.
The structure of the prior art is weaker. The receiving chamber has three open ends so that the structure is weaker. This will cause that the retaining jaw will be destroyed due to the force from the nut.
In another prior art structure, an L shape slot is used to replace the sector slot. However this makes the manufacturing process being tedious and complicated.
Furthermore, the when one end of the nut presses the driving roller to move inwards so that the space becomes larger and thus the spanner can rotate around the nut, but the inner side of the movable jaw has a flat surface. No space can receive the apex of the nut as the nut rotates so that the reverse rotation of the nut cannot be performed smoothly. Moreover in some prior arts, lateral sides of the retaining jaw and movable jaw for clamping a nut have respective flat surfaces. This will cause that the nut cannot rotate reversely.
Further, the opening of the receiving chamber causes dust to flow into the receiving chamber. As a result, the driving roller cannot move with a sufficient distance so that the function of driving a nut to rotate reversely is lost.